Polychlorinated biphenyls (PCBs) belong to a family of organic chemicals known as chlorinated hydrocarbons. They are commercially produced by the direct chlorination of biphenyl. Up to ten chlorine atoms may be attached at various locations on the biphenyl molecule which results in a possible 209 PCB compounds. PCBs are very stable and exhibit low water solubility, low vapour pressure, low flammability, high heat capacity and low electrical conductivity, making them extremely useful for a number of industrial purposes, including acting as an insulator in electrical capacitors and transformers. However, there are a number of adverse environmental and health effects associated with the use of PCBs. Earlier studies have shown that PCBs are the precursor molecules for a variety of environmentally hazardous compounds such as dioxin and furan. PCBs are known to cause malignant transformation of cells in humans. Such cellular toxic effects have also been demonstrated in experiments where laboratory animals have been repeatedly exposed to low levels of PCBs. The toxic effects of PCBs on both humans and animals together with the fact that PCBs excessively persist in the environment both point to the fact that PCBs have the potential to seriously affect man and the earth's ecosystems.
PCBs enter the environment in a number of ways. First, it has been estimated that more than half of the PCBs produced have been discarded in landfill sites and dumps. Contaminants washed out from such sites can enter ground water or are carried by rain and snow into nearby streams and rivers, finding their way into lakes and oceans. Second, incomplete combustion of PCB-containing wastes may release contaminants into the atmosphere in combustion gases that return to earth in rain or snow or as dry minute particles. Third, leaks from electrical transformers and capacitors seep into the soil, working their way into the environment. Fourth, liquids containing PCBs have been sprayed on roads in the past to act as a dust suppressant. Runoff from these roads drain into urban and rural sewage and drainage systems and into the environment. Finally, illegal dumping or disposal of PCBs by careless individuals or corporations results in contamination of the environment with PCBs.
The destruction of PCBs in the environment is the ultimate goal in PCB waste management, environmental cleanup and soil reclamation. Numerous technologies directed at effectively breaking down the molecular structure of PCBs into non-hazardous products have been developed. Two of these technologies, high-temperature incineration and chemical dechlorination, are currently being used on a commercial basis in a number of countries around the world. High temperature incineration, however, suffers from the drawback that it requires sophisticated and expensive equipment to perform the incineration. Chemical dechlorination requires the use of expensive chemicals and suffers from additional problems of disposal and environmental pollution from the resultant products from the chemical dechlorination.
A third technology, bioremediation, is an approach that can avoid further environmental problems and has advantages over incineration and chemical dechlorination in adaptability and cost. Bioremediation refers to the conversion of toxic environment contaminating compounds into innocuous substances by way of microbial digestion. Biodegradation of PCBs usually leads to the dechlorination of PCBs and formation of biphenyls and their derivatives. There has been much study on the use of bacteria for biodegradation of chlorinated biphenyls. It has been reported that certain strains of Alcalignes eutrophus and Pseudomonas sp. are effective for biodegradation of PCBs in river and lake sediments (Clark et al., 1979, Applied and Environmental Microbiology, Vol.37. No.4). However, several of these methods employed in these studies are not directed to a cost effective process which can be used on a large scale for the effective degradation of PCBs in soil.
U.S. Pat. No. 5,340,376 discloses a biological remediation process wherein microorganisms are employed to degrade contaminating organic compounds present within an environment. The process comprises applying a low-level of controlled-release source of microorganism nutrients capable of continuously supplying an effective microorganism growth and activity promoting level of nutrients over a period of at least two months. The controlled release nutrients are in the form of coated solid particles.
U.S. Pat. No. 4,391,887 discloses a process for the production of a storage-stable preparation of microorganisms capable of degrading products of industrial organic synthesis. The stabilised active mixed culture of microorganisms are treated with chemicals and then dried to obtain a storage-stable active mixed culture.
U.S. Pat. No. 5,401,413 discloses a composition for enhancing the biodegradation of biodegradable organic wastes which comprises the use of liposomes to supply essential nutrients for bacterial growth of microorganisms and at the same time, increase the polarity, wettability and availability of organic waste for enhancing bacterial interaction with the waste.
U.S. Pat. No. 4,493,895 discloses a method for the microbial degradation of halogenated organic chemical waste. The process comprises applying specifically designed strains of microorganisms capable of degrading the halogenated organic chemicals to carbon dioxide, water and salt. The microorganism strains have been engineered to contain a plasmid which confers the ability of decomposing halogenated organic chemicals.
While it is apparent that several bioremediation process exist for degrading toxic compounds in wastes, none of the processes are well adapted for large scale commercial use. Furthermore, none of the processes can achieve an optimal and desirable level for the degradation of PCBs in soil.
It is therefore an object of the present invention to provide an improved and efficient method for the microbial degradation of polychlorinated biphenyls in soil compared with other known waste decontamination procedures currently practiced today.